This invention relates to telephone systems including an analog cordless telephone system, a digital cordless telephone system and a portable radio telephone system, and more particularly to a cordless telephone system in which means are provided for controlling the process of charging a secondary battery built into a mobile telephone set or a portable radio telephone set and for establishing a quick communication mode ready for starting to originate an outgoing call or answering an incoming call as soon as the mobile telephone set or the portable radio telephone set is taken up from a charging stand.
In a cordless telephone system of this kind, its mobile telephone set is charged by loading it on a charging stand or a charging part of its stationary telephone set, so that it can operate with a quick communication mode when unloaded, as disclosed in, for example, JP-A-1-177748. The disclosure of the cited publication is such that, when the mobile telephone set is unloaded from the charging stand or from the charging part of the stationary telephone set, the voltage appearing at the charging terminal of the mobile telephone set when the mobile telephone set is unloaded from the charging stand or charging part changes from that appearing at the charging terminal when the mobile telephone set is kept loaded on the charging stand or charging part, and this voltage change is detected by a control part to permit starting to originate an outgoing call or answering an incoming call without pushing down the hook button of the mobile telephone set.
However, in the case of the prior art cordless telephone system, the voltage appearing at the charging terminal of the mobile telephone set when the mobile telephone set is kept loaded on the charging stand or on the charging part of the stationary telephone set and that appearing at the charging terminal when the mobile telephone set is unloaded from the charging stand or charging part are detected to determine the voltage change, and this voltage change is used to permit starting to originate an outgoing call or answering an incoming call without pushing down the hook button of the mobile telephone set. Therefore, when the supply of the charging voltage from the charging stand or from the charging part of the stationary telephone set is stopped due to an interruption of supply of commercial power from an AC power source or operational failure of a DC power supply supplying the DC voltage, for example, the internal control of the mobile telephone set is unable to accurately distinguish between the situation where power supply failure has occurred and the situation where the mobile telephone set is unloaded from the charging stand to permit starting to originate an outgoing call. Thus, the prior art cordless telephone system has had the problem of setting up a quick communication mode ready for starting to originate an outgoing call or answering an incoming call even in the event of an interruption of commercial power from the AC power source.